Asperity and head-media contact detection using multi-stage temperature coefficient of resistance sensor

ABSTRACT

A multi-stage sensor is situated on the head transducer and configured to interact with a magnetic recording medium. A first sensor stage of the multi-stage sensor has a temperature coefficient of resistance. A second sensor stage of the multi-stage sensor is coupled to the first sensor and has a temperature coefficient of resistance. The first sensor stage is configured to preferentially sense asperities of the media relative to the second sensor stage, and the second sensor stage configured to preferentially sense proximity to, and contact with, a surface of the media relative to the first sensor stage. The first and second sensor stages may be connected in series or in parallel.

RELATED PATENT DOCUMENTS

This application claims the benefit of Provisional Patent ApplicationSer. Nos. 61/414,733 and 61/414,734 both filed on Nov. 17, 2010, towhich priority is claimed pursuant to 35 U.S.C. §119(e) and which arehereby incorporated herein by reference in their respective entirety.

SUMMARY

Embodiments of the disclosure are directed to a temperature coefficientof resistance (TCR) sensor, and methods of using same, capable ofsensing specified features of a magnetic recording medium, such asfeatures of the magnetic recording medium having significantly differentscale. Embodiments of the disclosure are directed to a TCR sensor, andmethods of using same, having a multiplicity of sensor stages eachconfigured to sense a different feature of a magnetic recording medium,such as relatively small-scale features (e.g., features having a smallsurface area that interact with the TCR sensor) and relativelylarge-scale features (e.g., features having a large surface area thatinteract with the TCR sensor).

An apparatus, according to various embodiments, includes a headtransducer and a multi-stage sensor situated on the head transducerconfigured to interact with a magnetic recording medium. A first sensorstage of the multi-stage sensor has a temperature coefficient ofresistance. A second sensor stage of the multi-stage sensor is coupledto the first sensor and has a temperature coefficient of resistance. Thefirst sensor stage is configured to preferentially sense asperities ofthe media relative to the second sensor stage, and the second sensorstage configured to preferentially sense contact with a surface of themedia relative to the first sensor stage. According to some embodiments,the second sensor stage is connected in series with the first sensorstage. In other embodiments, the second sensor stage is connected inparallel with the first sensor stage. In further embodiments, the firstand second sensor stages can be operated independently, with each sensorstage having its own electrical connection pads.

Various embodiments are directed to a method involving use of amulti-stage TCR sensor situated on a head transducer. With the headtransducer moving relative to a magnetic recording medium, the methodinvolves preferentially sensing asperities of the medium using a firstsensor stage of the multi-stage sensor relative to a second sensor stageof the multi-stage sensor, and preferentially sensing contact with asurface of the medium using the second sensor stage relative to thefirst sensor stage. The method may further involve generating an outputsignal from the multi-stage sensor indicative of one or both of sensingasperities by the first sensor stage and sensing proximity to, andcontact with, the medium surface by the second sensor stage.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side view of a heater-actuated head transducerarrangement which incorporates a multi-stage TCR sensor in accordancewith various embodiments;

FIG. 2 is a front view of the heater-actuated head transducerarrangement shown in FIG. 1;

FIG. 3 shows the heater-actuated head transducer arrangement of FIGS. 1and 2 in a pre-actuated configuration and an actuated configuration;

FIG. 4A illustrates a representative temperature profile for aheater-actuated recording head transducer of the type shown in FIGS. 1-3before, during, and after contact between the head transducer and asurface of a magnetic recording disk;

FIG. 4B illustrates a representative temperature profile for anon-thermal actuatable recording head transducer before, during, andafter contact between the head transducer and a surface of a magneticrecording disk;

FIG. 5 is a flowchart showing various processes of a method fordetecting specified surface features of, and contact with, a magneticrecording medium using a multi-stage TCR sensor in accordance withvarious embodiments;

FIG. 6 shows a single-stage TCR wire sensor for illustrative purposes;

FIG. 7 shows a multi-stage TCR sensor with two different TCR sensorstages coupled in series in accordance with various embodiments;

FIG. 8A shows the static response of a large TCR wire sensor as afunction of heater power;

FIG. 8B shows the static response of a short TCR wire sensor as afunction of heater power;

FIGS. 9-12 show several configurations of a multi-stage TCR sensor withtwo different TCR sensor stages coupled in series in accordance withvarious embodiments;

FIG. 13 shows a multi-stage TCR sensor with two different TCR sensorstages coupled in parallel in accordance with various embodiments;

FIG. 14 is an airbearing surface view of the parallel multi-stage TCRsensor arrangement shown in FIG. 13; and

FIG. 15 is a graph of the relative response of two TCR sensor stageswired in parallel to two TCR sensor stages wired in series as a functionof the change of resistance in accordance with various embodiments.

DETAILED DESCRIPTION

Data storage systems commonly include one or more recording heads thatread and write information to a recording medium. It is often desirableto have a relatively small distance or spacing between a recording headand its associated media. This distance or spacing is known as “flyheight” or “head-to-media spacing.” By reducing the head-to-mediaspacing, a recording head is typically better able to both write andread data to and from a medium. Reducing the head-to-media spacing alsoallows for surveying of recording medium topography, such as fordetecting asperities and other features of the recording medium surface.

The ability to detect both head-to-media contact and head-to-asperitycontact is complicated by the competing objectives of detecting contactof two surface structures that differ significantly in terms of scale.Head-to-media contact, for example, is a relatively large contact eventinvolving a relatively large contact area. Head-to-asperity contact is arelatively small contact event involving a relatively small contactarea. Conventional sensing approaches typically use a single sensor forsensing both types of contact events, resulting in a compromised sensingscheme that is sub-optimal for sensing both types of contact events.

In accordance with various embodiments, and with reference to FIGS. 1-3,a slider 100 is shown supported by a suspension 101 in close proximityto a rotating magnetic storage disk 160. The slider 100 supports arecording head transducer 103 and a heater 102 thermally coupled to thehead transducer 103. The heater 102 may be a resistive heater thatgenerates thermal heat as electrical current is passed through theheater 102. The heater 102 is not limited to resistive heaters, and mayinclude any type of heating source. The thermal energy generated by theheater 102 causes thermal expansion of the head transducer 103. Thisthermal expansion can be used to reduce the head-to-media spacing 107 ina data storage system. It is noted that, in some embodiments, anon-thermal actuator can be used to reduce the head-to-media spacing107.

The multi-stage TCR sensor 105 is preferably configured to sense changesin heat flow for detecting asperities of the medium 160 andhead-to-media contact. Details concerning head-to-media spacing andcontact determinations in accordance with various embodiments of thedisclosure are provided in commonly owned U.S. Pat. No. 8,523,312 whichis incorporated herein by reference.

The multi-stage TCR sensor 105 is preferably configured to sense changesin heat flow for detecting asperities of the medium 160 andhead-to-media contact. Details concerning head-to-media spacing andcontact determinations in accordance with various embodiments of thedisclosure are provided in commonly owned U.S. patent application Ser.No. 12/941,461 filed Nov. 8, 2010 which is incorporated herein byreference.

As is depicted in FIG. 3, before head-to-media contact, there is an airgap 107 defined between the hot head surface and the relatively cooldisk 160. The head transducer 103, air gap 107, and magnetic recordingdisk 160 define one level of heat transfer rate. When the headtransducer 103 is in contact with the disk 160, such as after activationof the thermal actuator or heater 102, the direct contact between thehigh thermal conductivity materials of the head transducer 103 and thedisk 160 significantly increases the heat transfer rate. As such, theTCR sensor 105 on the head transducer 103 senses a drop of temperatureor an excursion of temperature trajectory, allowing for detection ofhead-to-media contact. As will be described in greater detailhereinbelow, the multi-stage TCR sensor 105 is preferably implemented toincorporate a multiplicity of sensor stages, each of which is sensitiveto disk surface features of different scale. In particular, amulti-stage TCR sensor 105 incorporates a sensor stage configured topreferentially sense proximity to, and contact with, a surface of themagnetic recording disk 160, and a sensor stage configured topreferentially sense asperities of the disk 160.

FIG. 4A illustrates a representative temperature profile for a recordinghead transducer 103 of the type shown in FIGS. 1-3 before, during, andafter contact between the head transducer 103 and a surface of themagnetic recording disk 160. In this illustrative example, thetemperature profile is represented as a steady state DC signal. When thehead transducer 103 is actuated by a thermal actuator 102, the headtransducer surface temperature will increase with the actuation due tothe heat generated by the thermal actuator 102. The head transducertemperature will be higher than the temperature of the disk 160. Assuch, the disk 160 acts as a heat sink in this scenario.

When the head transducer 103 contacts the disk 160, the head transducersurface temperature will drop due to a change in heat transfer rateresulting from the contact. The head transducer surface temperature willcontinue to increase due to thermal actuator heating and frictionalheating. The change in temperature or excursion in temperaturetrajectory can be used to declare head-to-media contact.

FIG. 4B illustrates a representative temperature profile for a recordinghead transducer 103 which is actuated by a non-thermal actuator. In thisillustrative example, the TCR sensor 105 bias power self-heats the TCRsensor to a temperature substantially higher than the temperature of thedisk 160. The disk 160 acts as a heat sink in this scenario. When thehead transducer 103 is actuated down toward the disk 160, the heattransfer rate increases gradually, which causes a gradual temperaturedecrease in the TCR sensor temperature. When the head transducer 103comes into contact with the disk 160, there will be a change in heattransfer rate, causing a head transducer surface temperature excursion.The TCR sensor 105 on the head transducer surface measures thistemperature excursion to detect head-to-media contact. Should furtheractuation into head-to-media contact occur, the temperature willeventually increase due to frictional heating.

As discussed previously, head-to-media clearance is an importantparameter that impacts magnetic disk recording performance. As the arealdensity increases, the head-to-media spacing decreases. As thehead-to-media spacing decreases, the importance of accurately measuringhead-to-media clearance, head-to-media contact, and head-to-asperitycontact increases. A multi-stage TCR sensor according to embodiments ofthe disclosure can be used to measure head-to-media clearance,head-to-media contact, and head-asperity contact. According to variousembodiments, a multi-stage TCR sensor incorporates a TCR resistivetemperature sensor comprising a wire that monitors the temperature andchange of temperature of the head transducer at the wire.

TCR wire sensors for asperity detection and contact detection havedifferent optimization paths. As summarized in Table 1 below, TCR wiresconfigured for asperity detection are typically designed to have hot(e.g., temperature of ˜100° C. above the transducer temperature) andsmall sensors. In general, hot sensors provide a good SNR. Small sensorsare able to determine the geometry of a small asperity for accuratetrack padding, for example.

TCR wires for head-to-media contact detection function better when theyare larger and have more of their sensing area at the airbearing surface(ABS). This allows such TCR wires to capture the transfer of heat fromthe airbearing surface to the media. Larger TCR wires have also beenshown to have an acceptable SNR at much lower temperatures (e.g., ˜10°C.). As such, it is not feasible to optimize a single device for bothasperity and contact detection.

TABLE 1 Sensor direction of goodness Asperity detection Contactdetection Cross track width Smaller Larger Temperature Hotter (~100° C.)Can be run cooler (~10° C.)

Turning now to FIG. 5, there is illustrated a flowchart showing variousprocesses of a method for detecting specified surface features of, andcontact with, a magnetic recording medium using a multi-stage TCR sensorin accordance with various embodiments. With a multi-staged sensor of ahead transducer moving 250 relative to a magnetic recording medium,contact with the media asperities detected 260 using a first TCR sensorstage of the multi-staged sensor. A signal indicative of contact betweenthe first TCR sensor stage and the media asperity is generated 262. Thissignal can be communicated town output of the multi-staged sensor.

As is further shown in FIG. 5, contact with the surface of the media isdetected 270 using a second TCR sensor stage of the multi-staged sensor.A signal indicative of surface contact is generated 272, which may becommunicated to an output of the multi-staged sensor. According to someembodiments, the first and second TCR stages are configured to operate255 in a series mode, such as by alternately changing bias powersupplied to the multi-staged sensor. In other embodiments, the first andsecond TCR stages are configured to operate 257 in a parallel mode, inwhich the first and second TCR stages may be operated alternately orconcurrently.

Embodiments of the disclosure are directed to a multi-stage resistivetemperature sensor comprising a TCR wire that has two elements.According to various embodiments, a TCR wire sensor includes a smallerhotter element for asperity detection and a larger, cooler element forcontact detection. A dual-stage TCR wire sensor, for example, includesboth a small hot element and a cooler large element. For purposes ofhighlighting particular features of a dual-stage TCR wire sensoraccording to various embodiments of the disclosure, reference will bemade to FIG. 6, which shows a single-stage TCR wire sensor 301. As wasdiscussed hereinabove, it is not feasible to implement a single-stageTCR wire sensor 301 that provides reliable detection of both asperityand media contact detection.

Although the single-stage sensor shown in FIG. 6 may have some limitedamount of material at the airbearing surface, a dual-stage TCR sensoraccording to embodiments of the disclosure has much more of the materialcreating the resistance of the element located at the ABS, where thetemperature gradient between the head transducer and disk is largest.Therefore, a larger change in resistance will be accompanied with ABSsurface temperature changes as compared to a conventional design thathas much more material located into the slider body, shielded from theABS surface.

FIG. 7 illustrates a dual-stage TCR sensor 302 which includes a firstsensor stage 335 (e.g., a hot TCR wire sensor) and a second sensor stage337 (e.g., a cold TCR wire sensor). The terms hot and cold associatedwith the first and second sensor stages 335 and 337 are used herein forpurposes of explanation, in view of the significantly differenttemperatures at which these two sensor stages typically operate (e.g.,˜100° C. and ˜10° C., respectively). The first sensor stage 335 issensitive to changes in heat flow across a small sensing area relativeto that of the second sensor stage 337. Accordingly, the first sensorstage 335 has greater sensitivity to changes in heat flow for detectingasperities of the magnetic recording medium. The second sensor stage 337is sensitive to changes in heat flow across a large sensing arearelative to that of the first sensor stage 335. As such, the secondsensor stage 337 has greater sensitivity to changes in heat flow fordetecting contact and spacing between the head transducer and thesurface of the magnetic recording medium.

In the embodiment shown in FIG. 7, the first and second sensor stages335 and 337 define a unitary sensing structure. The second sensor stage337 includes second sensor stage portions 337 a and 337 b, and the firstsensor stage 335 is situated between the second sensor portions 337 aand 337 b. In this configuration, the first and the second sensor stages335 and 337 are coupled in series. The first sensor stage 335 isconfigured to preferentially sense asperities of a magnetic recordingmedium, and the second sensor stage 337 is configured to preferentiallysense proximity to, and contact with, a surface of the magneticrecording medium. In other embodiments, the second sensor stage 337includes two spaced-apart portions 337 a and 337 b that are situated atspaced-apart locations on the airbearing surface. In such embodiments,the two spaced-apart portions 337 a and 3376 can be used to concurrentlymeasure contact with at least two spaced-apart locations of the surfaceof the medium.

According to some embodiments, when the dual-stage TCR sensor 302 is runin an asperity detection mode, a relatively large bias current can beused to significantly heat up the first sensor stage 335 to atemperature above ambient (i.e., above the disk temperature). Becauseasperity detection requires a hotter sensing element in comparison tomedia surface contact detection, a signal resulting from contact betweenthe dual-stage TCR sensor 302 and an asperity will only be detected whenthe asperity interacts with the smaller hot first sensor stage 335.Therefore, the size of the asperity can be determined more accurately bymeasuring the cross-track distance of the signal than when using a muchlarger sensor. When the dual-stage TCR sensor 302 makes contact with asurface of the magnetic recording medium, the signal output by thedual-stage TCR sensor 302 is a combined signal produced by both thesmall hot first sensor stage 335 and the much larger cooler secondsensor stage 337 that interacts with a significantly larger portion ofthe disk surface, thus resulting in a larger signal due to the largerarea heat transfer/influence.

FIGS. 8A and 8B show an illustrative example of such an effect. FIG. 8Ashows the static response of a large wire (˜10 μm long) as a function ofheater element power (5,000 μA corresponding to an overheat ratio (OHR)of ˜0.1). FIG. 8B shows the static response of a short wire (−0.5 μmlong) as a function of heater element power (1,600 μA corresponding toan OHR of ˜0.2). The plots of data in FIGS. 8A and 8B show that thelarger wire of FIG. 8A has a larger response with clearance at a lowertemperature than the smaller wire of FIG. 8B. It is noted that R0 iscalculated for each bias at zero heater power. Additional detailsconcerning wire lengths, heater element power, and correspondingoverheat ratios are provided in commonly owned U.S. Patent ApplicationPublication No. 2012/0120982, and U.S. Provisional Application Ser. No.61/414,733 filed on Nov. 17, 2010, each of which is incorporated hereinby reference.

With continued reference to FIG. 7, the first (hot) sensor stage 335 hasa cross-track length (HL—hot length), an into slider body depth (HD—hotdepth), and a down track width (HW—hot width). According to someembodiments, the first stage sensor 335 may have the following geometry:HL=750 nm; HD=75 nm; and HW=60 nm. Unlike a single-stage TCR wiresensor, such as that shown in FIG. 6, the dual-stage TCR sensor 302shown in FIG. 7 includes a significant amount of TCR material (e.g., amajority of the material) at the airbearing surface (ABS) to define acooler second (cold) sensor stage 337.

According to some embodiments, the second sensor stage 337 may have across-track length (CL—cold length) of about 15 μm, a down track width(CW—cold width) of about 1 μm, and an into slider body depth (CD—colddepth) of about 75 nm. It is noted that, although the respectivecross-track length and the down track width of the first and secondsensor stages 335 and 337 differ significantly (e.g., by a factor ofabout 20 and 17, respectively), the into slider body depth (HD and CD)of each of the first and second sensor stages 335 and 337 can be thesame. It is understood that the into slider body depths, HD and CD, ofthe first and second sensor stages 335 and 337 can be different.

According to various embodiments, the temperature of the hot firstsensor stage 335 can be controlled by changing the bias power of thesensor system (i.e., current, power, or voltage). The relative amount ofheat generated at the hot first sensor stage 335 compared to the heatgenerated at the cooler second sensor stage 337 can be controlled by thegeometry of the two sensor stages 335 and 337. That is, CL, CW, and CDcan be tuned to provide the desired relative sensitivity between the hotand cold sensor stages 335 and 337. For example, holding all otherdimensions fixed, as CW approaches HW, the temperature of the coldsecond sensor stage 337 will approach that of the hot first sensor stage335. The exact dimensions can be determined and selected (e.g.,optimized) based on the desired asperity and contact detectionsignal-to-noise ratio (SNR).

The multi-stage TCR sensor 302 includes a leading edge 340 and atrailing edge 350. Each of the first and second sensor stages 335 and337 has a respective leading edge and trailing edge that are alignedco-parallel with the leading and trailing edges 340 and 350 of themulti-stage TCR sensor 302. In the embodiment shown in FIG. 7, theleading edge of the first sensor stage 335 is recessed relative to theleading edge of the second sensor stage 337. The relative alignment andpositioning of the respective first and second sensor stages 335 and337, and the geometry of these sensor stages 335 and 337, may be variedto achieve specified asperity and media contact detection performancecharacteristics.

FIGS. 9-12 show different configurations of a series multi-stage TCRsensor in accordance with various embodiments of the disclosure. Themulti-stage TCR sensor 303 shown in FIG. 9, for example, includes a coldsecond sensor stage 337 having two second sensor stage portions 337 aand 337 b with opposing tapered edges contacting opposing ends of a hotfirst sensor stage 335. In the embodiment shown in FIG. 10, a hot firstsensor stage 335 is situated between two rectangular second sensor stageportions 337 a and 337 b, with the surface of the first sensor stage 335arranged co-planner with the leading edge 340 of the multi-stage TCRsensor 304.

According to the embodiment shown in FIG. 11, a multiplicity of hotfirst sensor stage elements 335 a, 335 b, and 335 c are situated betweentwo rectangular second sensor stage portions 337 a and 337 b, with asurface of two of the first sensor stage elements 335 a and 335 crespectively arranged co-planner with the leading and trailing edges 340and 350 of the multi-stage TCR sensor 305. In the embodiment of amulti-stage TCR sensor 306 shown in FIG. 12, the first sensor stage 335includes a multiplicity of first sensor stage, portions 335 a-335 n, andthe second stage sensor 337 includes a multiplicity of second sensorstage portions 337 a-337 n. In the configuration shown in FIG. 12, onefirst sensor stage portion (e.g., 335 b) is situated between a pair ofadjacent spaced-apart second stage portions (e.g., 337 b and 337 c). Asurface of each of the first sensor stage portions 335 a-335 n isarranged co-planar with the leading edge 340 of the multi-stage TCRsensor 306. Other configurations and arrangements of first and secondsensor stages and sensor stage portions are contemplated. As discussedpreviously, the specific arrangement and dimensions of the individualsensor element geometries can be defined from asperity and contactdetection SNR optimization algorithms.

In accordance with various embodiments of the disclosure, a multi-stageTCR sensor can be implemented to include a multiplicity of TCR sensorscoupled in parallel, with each TCR sensor configured to sense differentfeatures of a magnetic recording medium and/or different forms ofinteraction between the surface of the magnetic recording medium and themulti-stage TCR sensor. A multi-stage TCR sensor according to suchembodiments includes a first TCR sensor stage configured for sensinghead-to-asperity contact and a second TCR sensor stage configured forsensing head-to-media contact, with the first and second TCR sensorstages coupled in parallel. Implementations of a multi-stage TCR sensorwhich incorporates parallel connected asperity and contact TCR sensorsprovide for improved (e.g., optimized) geometry and electricalconnections, and account for design compromises that harmonize competingobjectives of head-to-asperity contact and head-to-media contactdetection.

FIG. 13 is an illustration of a multi-stage TCR sensor whichincorporates parallel connected asperity and media contact TCR sensorsin accordance with various embodiments. According to the embodimentshown in FIG. 13 the multi-stage TCR sensor 307 has a leading edge 340and a trailing edge 350. The multi-stage TCR sensor 307 includes a hotfirst sensor stage 335 having a surface that is arranged co-parallelwith the airbearing surface 320. As discussed previously, the firstsensor stage 335 is configured to be preferentially sensitive to mediasurface features of relatively small scale, such as asperities. Themulti-stage TCR sensor 307 further includes a cold second sensor stage337 having a surface that is arranged co-parallel with the airbearingsurface 320. The second sensor stage 337, also as discussed previously,is configured to be preferentially sensitive to media surface contact(i.e., media surface features of relatively large-scale).

The first and second sensor stages 335 and 337 shown in FIG. 13 areelectrically coupled in parallel, which is depicted by therepresentative wire connection 321. FIG. 14 is an airbearing surfaceview of the parallel multi-stage TCR sensor arrangement 307 shown inFIG. 13.

As has been described previously, the cold second sensor stage 337configured for head-to-media contact detection requires a relativelylarge area. As the heater actuated transducer head moves closer to themedia, there is a small flow of thermal energy from the contactdetection sensor stage 337 to the media. At contact, the thermaltransfer increases greatly, resulting in a lower temperature of thecontact detection sensor stage 337 and a subsequent resistance change.

The hot first sensor stage 335 configured for head-to-asperity contactdetection requires a small area relative to that of the contactdetection sensor stage 337. The asperity interacts directly with theasperity detection sensor stage 335 causing this sensor to increase ordecrease in temperature and resulting in a subsequent resistance change.The temperature increases if the asperity has been pre-heated by rubbingon the upstream airbearing surface 320. The temperature decreases if therelatively colder asperity has had minimal contact with the transducerhead before interacting with the asperity detection sensor stage 335.

For purposes of illustration, and not of limitation, it is assumed thatthe response of the parallel multi-stage TCR sensor 307 shown in FIGS.13 and 14 is proportional to the change in resistance. In addition, itis assumed that the resistances are at operating conditions. Theresistance, R, for each of the first sensor stage 335 (R_(AD)) andsecond sensor stage 337 (R_(CD)) increases with increased current.

For a multi-stage TCR sensor wired in series, such as the TCR sensors302-306 shown in FIGS. 7, and 9-12:R ₀ =R _(CD) +R _(AD)where R₀ is the initial resistance, R_(CD) is the contact detectionsensor stage resistance, and R_(AD) is the asperity detection sensorstage resistance. For a multi-stage TCR sensor wired in parallel, suchas the TCR sensor 307 shown in FIGS. 13 and 14:

$\frac{1}{R_{0}} = {\frac{1}{R_{CD}} + \frac{1}{R_{AD}}}$$R_{0} = \frac{R_{CD}R_{AD}}{R_{CD} + R_{AD}}$

For asperity detection, the transducer head is kept at constantclearance. Consequently, there will not be any additional heater-inducedthermal changes in the two sensor stages. It is assumed that theinteraction with the asperity only affects the resistance of theasperity detection sensor stage. As such, the change in detectedresistance becomes:

Wired in series:R ₁ =R _(CD)+(R _(AD) +ΔR)and the percentage change in resistance is given by:

$\frac{R_{1} - R_{0}}{R_{0}} = {\frac{\left( {R_{CD} + R_{AD} + {\Delta\; R}} \right) - \left( {R_{CD} + R_{AD}} \right)}{R_{CD} + R_{AD}} = \frac{\Delta\; R}{R_{CD} + R_{AD}}}$Wired in parallel:

$R_{1} = \frac{R_{CD}\left( {R_{AD} + {\Delta\; R}} \right)}{R_{CD} + R_{AD} + {\Delta\; R}}$$\frac{R_{1} - R_{0}}{R_{0}} = {\frac{\frac{R_{CD}\left( {R_{AD} + {\Delta\; R}} \right)}{R_{CD} + R_{AD} + {\Delta\; R}} - \frac{R_{CD}R_{AD}}{R_{CD} + R_{AD}}}{\frac{R_{CD}R_{AD}}{R_{CD} + R_{AD}}} = \frac{R_{CD}\Delta\; R}{R_{AD}\left( {R_{CD} + R_{AD} + {\Delta\; R}} \right)}}$

For small changes in resistance, the response for a multi-stage TCRsensor wired in parallel is R_(CD)/R_(AD) times the response for the TCRsensor stages wired in series. Consequently, if R_(CD)>R_(AD), theresponse for asperity detection will be greater for the TCR sensorstages wired in parallel.

FIG. 15 is a graph of the relative response of the two TCR sensor stageswired in parallel to the two TCR sensor stages wired in series as afunction of the change of resistance. FIG. 15 shows how the ratio of theparallel-to-series TCR sensor signal behaves as a function of the changein resistance for the asperity detection sensor stage. The graph of FIG.15 shows that, for small changes in resistance due asperity detection,the relative response for the parallel TCR sensor circuit is greaterthan the response for the series TCR sensor circuit when R_(CD)>R_(AD).

As the resistance change increases, the response in the parallel TCRsensor circuit reduces relative to that of the series TCR sensorcircuit. FIG. 15 also shows that the asperity can cool the asperitydetection sensor stage, resulting in a lower resistance. In the case ofa cooling asperity, wiring the two sensor stages in parallel has an evengreater benefit.

For contact detection, the transducer head is pushed closer to the mediausing the thermal actuator, or other actuator device, untilhead-to-media contact is detected. With a thermal actuator, theresistances of both TCR sensor stages is gradually increasing. Forpurposes of simplicity, only the change in resistance immediately beforeand at contact is considered. Consequently, complications due to thethermal actuator are ignored.

For contact detection, both TCR sensor stages experience the change inthermal load. To first order, both TCR sensor stages will have the sameproportional response, β. It is noted that, because the relatively coolmedia reduces the temperature of the TCR sensor stages, β is negative.

In the case of the two TCR sensor stages wired in series:R ₁ =R _(CD)(1+β)+R _(AD)(1+β)and the percentage change in resistance is given by:

$\frac{R_{1} - R_{0}}{R_{0}} = {\frac{{\left( {R_{CD} + R_{AD}} \right)\left( {1 + \beta} \right)} - \left( {R_{CD} + R_{AD}} \right)}{R_{CD} + R_{AD}} = \beta}$In the case of the two sensor stages wired in parallel:

$R_{1} = \frac{R_{CD}{R_{AD}\left( {1 + \beta} \right)}}{R_{CD} + R_{AD}}$$\frac{R_{1} - R_{0}}{R_{0}} = {\frac{\frac{R_{CD}{R_{AD}\left( {1 + \beta} \right)}}{R_{CD} + R_{AD}} - \frac{R_{CD}R_{AD}}{R_{CD} + R_{AD}}}{\frac{R_{CD}R_{AD}}{R_{CD} + R_{AD}}} = \beta}$

Consequently, the contact detection response for a multi-stage TCRsensor is the same for the two sensor stages wired in series orparallel. According to various embodiments, the resistance of thecontact detection sensor stage (R_(CD)) may be in the range of about 1.5to 4 times greater than the resistance of the asperity detection sensorstage (R_(AD)).

In addition to the various series and parallel multi-stage TCR sensorembodiments described hereinabove, other multi-stage TCR sensorconfigurations are contemplated. According to some embodiments, forexample, a multi-stage TCR sensor may be implemented to include a hotfirst sensor stage and a cold second sensor stage, with each of thefirst and second sensor stages having its own electrical connection padsand operating independently.

It is to be understood that even though numerous characteristics ofvarious embodiments have been set forth in the foregoing description,together with details of the structure and function of variousembodiments, this detailed description is illustrative only, and changesmay be made in detail, especially in matters of structure andarrangements of parts illustrated by the various embodiments to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. An apparatus, comprising: a head transducer; anda multi-stage sensor situated on the head transducer to interact with amagnetic recording medium, the sensor comprising: a first sensor stagehaving a temperature coefficient of resistance; and a second sensorstage coupled to the first sensor, the second sensor stage having atemperature coefficient of resistance; the first sensor stage configuredto preferentially sense asperities of the medium relative to the secondsensor stage; and the second sensor stage configured to preferentiallysense contact with a surface of the medium relative to the first sensorstage.
 2. The apparatus of claim 1, wherein the second sensor stage isconnected in series with the first sensor.
 3. The apparatus of claim 1,wherein the first sensor stage operates at a temperature higher thanthat of the second sensor stage.
 4. The apparatus of claim 1, wherein:the first sensor stage is sensitive to changes in heat flow across asmall sensing area relative to that of the second sensor stage; and thesecond sensor stage is sensitive to changes in heat flow across a largesensing area relative to that of the first sensor stage.
 5. Theapparatus of claim 1, wherein: the first sensor stage has greatersensitivity to changes in heat flow for detecting asperities of themedium relative to the second sensor stage; and the second sensor stagehas greater sensitivity to changes in heat flow for detecting contact ornear-contact with the medium surface relative to the first sensor stage.6. The apparatus of claim 1, wherein the first sensor stage is smallerthan the second sensor stage.
 7. The apparatus of claim 1, wherein: thefirst sensor stage has a length, a width, and a depth; and the secondsensor stage has a length, a width, and a depth, where the length andthe width of the second sensor stage is greater than the length and thewidth of the first sensor stage.
 8. The apparatus of claim 7, whereinthe respective depths of the first and second sensor stages differ orare the same.
 9. The apparatus of claim 1, wherein the first and secondsensor stages are configured to operate alternately.
 10. The apparatusof claim 1, wherein the first and second sensor stages are configured tooperate alternately in response to changing of a bias power supplied tothe sensor.
 11. The apparatus of claim 1, wherein the first and secondsensor stages define a unitary structure, and at least a portion of thefirst sensor stage is situated between portions of the second sensorstage.
 12. The apparatus of claim 1, comprising a slider that supportsthe head transducer and comprises an airbearing surface, wherein amajority of material defining the multi-stage sensor is at theairbearing surface.
 13. The apparatus of claim 1, wherein; the first andsecond sensor stages each comprise a leading edge and a trailing edge;and the leading edge of the first sensor stage is recessed relative tothe leading edge of the second sensor stage.
 14. The apparatus of claim1, wherein the first sensor stage comprises a plurality of spaced-apartfirst sensor stage portions.
 15. The apparatus of claim 1, wherein thefirst sensor stage comprises a plurality of spaced-apart first sensorstage portions, and the second sensor stage comprises three or morespaced-apart second sensor stage portions.
 16. The apparatus of claim 1,wherein the second sensor stage is connected in parallel with the firstsensor stage.
 17. The apparatus of claim 16, wherein a resistance of thesecond sensor stage is greater than that of the first sensor stage. 18.The apparatus of claim 16, wherein a resistance of the second sensorstage is greater than that of the first sensor stage by a factor ofbetween about 1.5 to about
 4. 19. The apparatus of claim 16, wherein thesecond sensor stage comprises at least two spaced-apart portionsconfigured to concurrently measure contact with at least twospaced-apart locations of the surface of the medium.
 20. The apparatusof claim 1, wherein each of the first and second sensor stages isconfigured to operate independently.
 21. A method, comprising: with ahead transducer, comprising a multi-stage sensor, moving relative to amagnetic recording medium, preferentially sensing asperities of themedium using a first sensor stage of the multi-stage sensor relative toa second sensor stage of the multi-stage sensor; preferentially sensingproximity to, and contact with, a surface of the medium using the secondsensor stage relative to the first sensor stage; and generating anoutput signal from the multi-stage sensor indicative of one or both ofsensing asperities by the first sensor stage and sensing contact withthe medium surface by the second sensor stage.
 22. The method of claim21, wherein the first and second sensor stages are coupled in series.23. The method of claim 21, wherein the first and second sensor stagesare coupled in parallel.
 24. The method of claim 21, wherein each of thefirst and second sensor stages is configured to operate independently.